The isolation of substances from plant and animal sources which affect the growth and differentiation of normal and malignant human cells is paramount in the study of the processes which control cellular activities. By studying these purified materials as biochemical tools, we may be able to understand the biochemical mechanisms controlling cell proliferation and differentiation. Also, by investigating points where control of metabolic pathways are possible, we may find chemical agents to control cellular growth, differentiation, and proliferation. We have clarified the problem of the specificity of isolectins from Phaseolus vulgaris phytohemagglutinin in this phase of the studies.